The thickness of the slag in a refining crucible is important because it shows how far the refining operation has progressed. Furthermore its upper level must be monitored to prevent the crucible from overflowing. To this end my earlier U.S. Pat. No. 4,098,128 describes a system wherein an acoustic tube having several resonant frequencies is directed into the mouth of the refining crucible and is heated to above 100.degree. C. A microphone at the remote end of this tube receives sound passing through the tube and generates an electrical output corresponding to the sound received. This electrical output is delinearized over a band width including several of the resonant frequencies of the tube and the signal is then passed through a low-pass filter to eliminate parasitic oscillations and through a mixer where it is combined with a signal from a local oscillator. The intermediate frequency thus produced is passed through either of a pair of band-pass filters and is displayed. The signal strength of the output is inversely proportional to the depth of slag in the crucible, as the deeper the slag the more sound is absorbed by it.
The original source of the sound that is measured by this system is from the oxygen lance also directed at the surface of the melt in the crucible. The information obtained not only indicates the depth or thickness of the slag layer, but also its consistency.
Normally the acoustic tube with its microphone is mounted outside the crucible so as to reduce as much as possible the danger of it becoming damaged by material bubbling up in the melt. Thus such an arrangement is only usable adjacent a single crucible to which oxygen is being fed via its respective lance. In a plant where several such crucibles are operated simultaneously adjacent one another the above-described system operates imperfectly as it picks up noises in the appropriate frequency range from the adjacent crucibles.
The above-described system also has the disadvantage when used with a top-blown vessel having a hood such as described on pages 486 ff of The Making, Shaping and Treating of Steel edited by H. McGannon (Herbick & Held: 1971) that it is practically impossible to provide a safe mount for the acoustic tube and microphone. The hood must form a more or less tight joint with the stack and with the top of the crucible, normally by means of a skirt closed around the top of the crucible by hydraulic cylinders. In such systems it is essential to prevent air from entering the crucible during the blow as well as to prevent the dangerous post-combustion gases, such as carbon monoxide, from leaking out during the refining operation. This hood is normally provided with, or is in fact formed by, an array of tubes through which water is circulated to keep exhaust-gas temperatures down and to prevent the often chemically active stack gases from reacting with and damaging the hood.